Speed control for ward leonard systems



Dec. 23, 1947. E. u. I Assl-:N 2,433,130

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SPEED CONTROL FOR WARD LEONARD SYSTEMS Filed Nov. 3, 1945 2 sheets-sheet 2 @nga Patented Dec. 23, 1947 SPEED CNTROL FOR WARD LEONARD SYSTEMS Eivind U. Lassen, Whitesh Bay, Wis., assignor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application November 3, 1945, Serial No. 626,514

3 Claims. (Cl. 318-154) This invention relates to speed regulating systems for electric motors.

The invention relates more particularly to variable voltage drives of the so-called Ward Leonard type in which a driving `motor iS supplied with current by a variable voltage generator and is used to drive a load of high inertia such as a centrifugal casting machine.

The patent to Jesse E. Jones, No, 2,281,844, issued May 5, 1942, discloses a control system for drives of the aforesaid type including field regulating means which is controlled in accordance with the difference in the output voltage of an adjustable master potentiometer, and the output voltage of a tachometer generator coupled to the driving motor to effect acceleration or deceleration of the driving motor to speeds which are preset on the master potentiometer. In this systern the field regulating means operates t effect acceleration and deceleration of the driving motor at a uniform rate regardless of current conditions in the supply circuit of said motor. However it has been found that where the motor is used to drive a load of high inertia such as a centrifugal casting machine it is necessary to vary the rate of operation of the eld regulating means so as to prevent excessive current values in the motor circuit during starting and also during regenerative braking operations.

The present invention has among its objects to provide a system of the type disclosed in the aforementioned patent to Jesse E. J ones" in which the operation of the lield regulating means is not only controlled by differences in the output voltage of the master potentiometer and the tachometer generator but is also controlled by current conditions in the armature circuit of the driving motor to limit the current in such circuit during starting of the motor and also during regenerative braking thereof.

Another object is to provide a system of the aforesaid type which provides for a drifting stop of the motor and which also provides for slow down of the motor by regenerative braking at any point during a drifting stop.

Another object is to provide a system of the aforesaid type in which current in the armature circuit of the motor is maintained below a given limit for a predetermined interval upon starting of the motor and is thereafter permitted to rise to a higher predetermined limit for acceleration of the motor to a preset speed.

Another object is to provide for automatic return of the field regulating means to olf position during a drifting stop or a regenerative braking stop of the motor and to also regulate the rate of return of the eld regulating means toward off position so that the motor is not subjected to excessive regenerative braking current.

Various other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate a motor control system embodying the invention which will now be described, it being understood that the system illustrated is susceptible of modication without departing from the spirit and scope of the appended claims.

In the drawings,

Figure 1 is an across-the-line diagrammatic representation of a control system embodying the invention, and

Fig. 2 is a key illustration of certain switches and relays employed in Fig, 1.

The illustration of the switches and relays in Fig. 2 shows them with their coils and contact members disposed in horizontal alignment with their positions in the across-the-line circuits of Fig. 1 so Athat the reader may readily determine the position of the coil and contact membersof any switch or relay in the across-the-line circuit. The reference characters also serve as a guide to the relation between the operating coils of the switches and relays and their associated contacts. For example, switch MS is provided with an operating coil M1, normally open contacts MS2, M83 and M64, and normally closed contacts MS5 and MSS.

In the following description of the control system the switches and relays illustrated in Fig. 2 are named as follows:

MSmain switch.

IAR, ZAR-accelerating relays. DRf-decelerating relay. TRf-timing relay.

I CIR-starting relay. 2GB-braking relay.

IVR, 2VR-voltage relays. S-slow switch.

F-fast switch.

Referring to Fig. 1, the same illustrates a motor M which may be used to drive machines of various types, as for example a centrifugal casting machine which is of high inertia. It is desired to preset the operating speed of motor M either while the same is at rest or in operation and to subsequently cause the speed thereof to change to the preset speed and to be maintained at such speed. As hereinafter set forth, the speed of motor M is preset on a master potentiometer 9 having a resistor 9e which is adjustable by a slider Sb and is connected across a direct current supply circuit indicated by lines L1, L2.

In the system illustrated motor M is supplied with current by a variable voltage generator G which is driven at a substantially constant speed by a suitable prime mover (not shown) such as an induction motor. Motor M is provided with an armature MA and a separately excited field winding MF which is connected across lines L1, L2 in series with a rheostat IB. Generator G is provided with an armature GA and a separately eX- cited eld winding GF which is connected across lines L1, L2 in series with a rheostat Rheostat I8 is provided with a resistor IIBa which is adjustable by a movable contact IGI), and rheostat is provided with a resistor ||a which is adjustable by a movable contact IIb. Contacts Illb and Ib are carried by a rheostat arm I2 which is driven by a reversible motor I3 and resistors Iiia and I Ia are so arranged that upon movement of said arm toward the right away from the on position shown in Fig. 1 resistor ||a is rst eX- cluded from the eld circuit of generator G and thereafter resistor |11"l is included in the field cirf cuit of motor M. Arm I2 also has a bridging contact I4 associated therewith and as hereinafter set forth said bridging contact cooperates with stationary contacts |42, I4b and I4c to control energizing circuits for starting relay ICR and main switch MS.

Motor I3 is supplied with current from lines L1, L2 and is provided with an armature I3a and a series field winding |3b. The direction of 0peration of motor I3 is controlled by switches F and S, the former switch providing for operation of said motor in a direction to move contacts IGI and ||b toward the right for acceleration of motor M,` and the latter providing for operation of said contacts toward the left for deceleration of said motor. Switches F and S are selectively controlled by a polarized relay I5 which is responsive to differences in potential between slider 9b of the master potentiometer 9 and a tachometer generator TG. Generator TG is coupled to motor M to rotate therewith so that its output voltage is a direct function of the speed of said motor.

Relay I5 is provided with a pivoted contact arm I6 which is movable in one direction out of an intermediate off position to engage a cooperating contact IScl for establishment of an energizing circuit for switch S, and in an opposite direction out of off position to engage a cooperating stationary contact |61 for establishment of an energizing circuit for switch F. Relay I5 is provided with a pair of windings I'iF* and I'lb which are connected across lines L1, L2 and act to exert balanced opposing torques on contact arm IS. Said relay is also provided with series connected actuating windings I8,a and lh which are wound in opposite directions with respect to each other so that current passing therethrough causes movement of contact arm I8 out of oiposition in one direction or the other depending upon the direction of the current in said coils.

One terminal of tachometer generator TG is connected to line L2 and as hereinafter set forth upon starting of motor M the coils I8 and I8b of relay I5 are connected in series between the opposite terminal of said generator and slider 9b of master potentiometer 9. Assuming that motor M is operating at a speed below the speed preset on master potentiometer 9, current will flow from slider 9b through coils |82 and I8b to generator TG to maintain contact arm I6 in engagement with contact I61D for energization of switch F until motor M is accelerated to the preset speed. On the other hand, assuming that motor M is operating at a speed above the speed preset on master potentiometer 9, current will flow from generator TG through coils I8a and I8b to slider 9b to maintain contact arm I6 in engagement with contact |62 for energization of switch S until motor M is decelerated to .the preset speed.

As hereinafter set forth, switch F is controlled by accelerating relays IAR and ZAR to regulate the rate of acceleration of motor M, and switch S is controlled by decelerating relay DR to regulate the rate of deceleration of said motor, The Operating windings of these relays are connected in series in the supply circuit between motor armature MA and generator armature GA. Also as hereinafter set forth, timing relay TR operates after a predetermined interval upon starting of motor M to by-pass accelerating relay IAR.

Switches F and S are also controlled by limit switches FL and SL, respectively. Limit switch SL provides for deenergization of switch S upon movement of rheostat arm I2 into the extreme off position shown in Fig. l, and limit switch FL provides for deenergization of switch F upon movement of said arm into its opposite extreme position.

In addition to the aforementioned switches and relays the control system includes a normally open start push button switch 2D, a normally closed stop push button switch 2|, a normally closed drift push botton switch 22 and a normally open brake push button switch 23. As will now be set forth, start push button switch 20 operates through the medium of relay ICR to effect starting of motor M, and stop button switch 2| operates through the medium of said relay to effect stopping of said motor by regenerative braking. Also as will now be set forth, drift push button switch 22 provides for control of main switch MS and starting relay ICR to bring motor M t0 a drifting stop, and brake push button switch 23 operates through the medium of relay 2CR to effect slow down of motor M by regenerative braking at any time after a drifting stop is initiated by the drift push button switch 22.

When motor M is stopped, rheostat arm I2 is in the on position shown in Fig. 1 and all of the control switches and relays with the exception of timing relay TR are deenergized. The operating winding TR1 of timing relay TR is then connected across lines L1, L2 through the medium of main switch contacts MSG to hold said relay in open position.

To effect starting of motor M start push button switch 20 is depressedl and the operating winding ICRl of starting relay ICR is then connected across lines L1, L2 through the medium of drift push button switch 22, rheostat contacts |43, I4 and |412, and stop push button switch 2|. Relay ICR then responds andthe operating winding MS1 of main switch MS is energized by current from lines L1, L2 through thev medium of drift push button switch 22, rheostat contact I4a and starting relay contacts ICR4. Upon response of main switch MS a low resistance loop circuit is established between the generator armature GA and the motor armature MA by main switch contacts MS2 to provide for starting of motor M. At this time rheostat resistor II2L is included in the iield circuit of generator G, and rheostat resistor |02 is excluded from the eld circuit of motor M for operation of said motor at its lowest speed.

Immediately upon response of starting relay ICR the actuating windings I8a and Iiilo of relay I5 are connected in series between tachometer generator TG and slider 9b of master potentiometer 9 by starting relay contacts ICR2. Assuming that slider Sb is preset for operation of motor M at some intermediate speed or at full speed, current will 110W from slider Sb through -coils I8D and I8a to the tachometer generator TG to maintain contact arm Iii in engagement with its associated stationary contact Ib. Switch F will then be energized by a circuit extending irom line Ll through main switch contacts MSS, through contact arm I6 and its associated stationary contact Ib, through accelerating relay contacts IAR2 and 2AR2, through the operating winding F1 of switch F and through limit switch FL to line L2. Upon response of switch F motor i3 is connected across lines L1, L2 for operation in a direction to ad- Vance rheostat arm I2 toward the right for acceleration of motor M.

Upon release of start push button switch 2i! starting relay ICR is maintained energized through the medium of drift push button switch 22, starting relay contacts ICR5 and stop switch 2l. Also upon movement of rheostat arm I2 out of 01T position main switch MS is maintained energized through the medium of drift push button switch 22, rheostatV contacts Ilia, I4 and Idc and main switch contacts MS4.

Upon starting of motor M the rate of acceleration thereof is rlrst controlled -by accelerating relay IAR and thereafter by accelerating relay ZAR. Relay IAR responds to a given high .current Value in the armature circuit of motor M and upon response of said relay, contacts IAR2 are opened and switch F is thus deenergized to stop accelerating operation of rheostat arm I2. When the current in the armature circuit of motor M subsides accelerating relay IAR drops out and switch F is again energized for continued accelerating operation of rheostat arm I2.

As hereinbefcre stated, prior to starting of motor M the operating winding TR1 of timing relay TR is connected across lines L1, L2 by main switch contacts MSS. Upon starting of motor M timing relay TR is deenergized by opening of main switch contacts MS6 but dropping out of said relay is delayed for a predetermined interval by a resistance 1' and condenser c which are connected in series across the terminals of its operating winding TR1. Upon dropping out of relay TR the contacts TR2 thereof close to bypass accelerating relay contacts IAR2 and switch F is then controlled by contacts 2AR2 of accelerating relay ZAR.. Accelerating relay 2AR is set to respond at a higher current value than accelerating switch IAR. Thus upon dropping out of timing relay TR switch F is controlled by accelerating relay ZAR to control accelerating operations of rheostat arm I2 for an increased rate of acceleration of motor M.

When motor M is brought up to the speed preset on master potentiometer 9 no current will iiow through the actuating windings I8EL and I8b of relay I5. Contact arm I6 then returns to ofi position and switch F is thus deenergized to stop rheostat arm I2 in the position it then occupies for continued operation of motor M at the preset speed.

Assuming that motor M is operating at some preset speed and that slider 9b of the master potentiometer is moved to a position for a higher speed. Relay I5 will again operate as hereinbefore set forth to energize switch F for acceleration of motor M until said motor is brought up to the speed corresponding to the setting of the slider 9b. On the other hand, assuming that motor M is operating at a preset speed and that slider 9b of master potentiometer 9 is moved to a selected position to reduce the speed thereof. Current will then flow from tachometer generator TG through actuating coils I8@ and I8b of relay I5 to the slider 9b and contact arm I6 will be moved into engagement with its associated stationary contact Ia. The operating winding S1 of switch S will then be connected across lines L1, L2 through the medium of main switch contacts MS2, decelerating relay contacts DR2 and limit switch SL. Switch S then responds and motor I3 is connected across lines L1, L2 for operation in a direction to move rheostat arm I2 toward orf position for slowdown of motor M by regeneration, Decelerating relay DR is set to respond if the regenerated current exceeds a predetermined value, and in responding said relay opens its contacts DR2 to deenergize switch S. Slowdown operation of rheostat arm I2 is thus stopped until the regenerated current drops suiciently to permit dropping out of relay DR, whereupon switch S is again energized for further slowdown operation of rheostat arm I2. When motor M is slowed down to the speed selected on potentiometer 9 no current will flow in windings IZIa and Iiib. Contact arm I6 then returns to off position and switch S is deenergized to stop rheostat arm I2 in the position it then occupies for continued operation of Motor M at the selected speed.

Assuming now that motor M is operating at some preset speed and that it is desired to effect quick stopping thereof by regenerative braking. Stop push button switch 2l is operated to interrupt the aforedescribed maintaining circuit for starting relay ICR. Main switch MS is maintained energized through the medium of drift push button switch 22, rheostat contacts |42, I4 and I4c and main switch contacts MS2, and upon dropping out of starting relay ICR switch S is connected across lines L1, L2 through the medium of main switch contacts MS3, starting relay contacts ICR3, decelerating contacts DR2 and limit switch SL. Switch S is thus energized to return rheostat arm I2 toward off position for slowdown of motor M by regeneration. During return of' rheostat arm I2 toward orf position switch S is controlled as hereinbefore set forth by decelerating relay DR to temporarily stop operation of said arm when the regenerated current exceeds a given value. Upon movement of rheostat arm I2 into oi position the aforedescribed maintaining circuit for main switch MS is opened by disengagement of rheostat contact i4 from its associated stationary contact I4c and the energizing circuit for switch S is opened by limit switch SL. Motor M is thus stopped and the system is resetfor starting of said motor by start push button switch 20.

It is sometimes desirable to slow down motor M without regenerative braking by merely letting the same drift to a stop. For this purpose drift push button 2t] is depressed to open the aforedescribed maintaining circuits for main switch MS and starting relay ICR. Upon dropping out of main switch MS the operating winding of voltage relay winding ZVR is included in the loop circuit between generator armature GA and motor armature NA by opening of contacts MS2 and contact arm I of voltage relay I is disconnected from line L1 by opening of contacts MS3 to render said relay ineffective. Only a very small amount of current can iiow through voltage relay winding ZVRl and motor M will therefore drift until the same comes to a. stop. Upon slowing down of motor M the counter voltage thereof drops below the Voltage of generator G. Relay 2VR then responds and the operating winding Sl of switch S is connected across lines L1, L2 through the medium of voltage relay contacts ZVRT, decelerating relay contacts DB2 and limit switch SL. Motor I3 is thus energized to return rheostat arm I2 toward on position and when the generator voltage becomes approximately the same as the counter voltage of motor M voltage relay ZVR. drops out to stop operation of said arm. Relay 2VR will therefore operate intermittently during a drifting stop to maintain the generator and motor voltages at approximately the same value until rheostat arm I2 is returned to off position. This enables proper slow down of the motor M by regenerative braking at any time during a drifting stop by operation of brake push button 23.

The operating winding IVRl of voltage relay IVR is connected across the armature of motor M and during a drifting stop said relay is maintained in closed position. Upon depression of the brake push button switch 23 the operating winding 2CRl of braking relay ZCR, is connected across lines L1, L2 through the medium of main switch contacts MS5 and voltage relay contacts IVR2, and upon release of said brake push button switch said relay is maintained energized through the medium of its associated contacts 20E-3. When the counter voltage of motor M is approximately the same as the voltage of generator G relay ZVR is in normal position and upon closure of braking relay 2GB main switch MS is energized by a circuit extending from line L1 through starting relay contacts ZCRS, main switch contacts MS5, voltage relay contacts IVR2, starting relay contacts 2CR2, voltage relay contacts 2VR2 and through said main switch winding MSl to line L2. Main switch MS then responds and the loop circuit between generator armature GA and motor armature MA is reestablished by contacts MS2 for slow down of the motor by regenerative braking. Upon response of main switch MS braking relay 2CR is deenergized by opening of main switch contacts MS5 and said main switch is then maintained energized through the medium of its associated contacts MSl. Also upon response of main switch MS circuit is reestablished from line L1 to contact arm IG through the medium of main switch contacts MS3. Switch S is then controlled as hereinbefore set forth through the medium of decelerating relay DR for return of rheostat arm l2 toward off position. Main switch MS is deenergized upon return of rheostat arm I2 to on" position, but as is apparent, prior to return of rheostat arm I2 to off position an additional drift period can be started by depression of drift push button switch 22.

What I claim as new and desire to secure by Letters Patent is:

1. The combination with a direct current motor and a direct current generator for supplying current thereto, of a normally open main switch for establishing a low resistance supply circuit between said motor and said generator, an adjustable field regulator movable out of a given normal position to strengthen the field of said generator for acceleration oi said motor and returnable toward normal position to weaken the eld ci said generator for deceleration of said motor by regenerative operation thereof, a tachometer generator coupled to said motor for Supplying a voltage which is a function of the speed of said motor, a speed setting potentiometer for supplying an adjustable output voltage, means rendered operative upon closure of said main switch and responsive to voltage diierentials between said tachometer and said speed setting potentiometer for automatically co-ntrolling Said eld regulator to effect operation of said motor at speeds determined by the setting of said potentiometer, and means associated with said last mentioned means for temporarily arresting operation of said iield regulator during accelerating or decelerating operations thereof in response to predetermined high current conditions in said supply circuit.

2. The combination with a direct current motor and a direct current generator for supplying current thereto, of a normally open main switch for establishing a low resistance supply circuit between said motor and said generator, a speed selecting device, an adjustable eld regulator rendered operative upon closure of said main switch for automatically controlling the field of said generator for operation of said motor at speeds determined by the setting of said speed selecting device, said neld regulator being movable out of a given normal position to strengthen the field of said generator for acceleration of said motor and being returnable to normal position to weaken the field of said generator for deceleration of said motor by regenerative braking operation thereoi, manually controlled means for opening said main switch for a drifting stop of said motor and ior reclosing the same at any time during a drifting stop for slowdown of said motor by regenerative operation thereof, and means rendered operative upon opening of said main switch for a drifting stop to automatically return said eld regulat-or toward normal position at a rate to prevent substantial variation between the output voltage of said generator and the counter E. M. F. of said motor through the drifting period.

3. The combination with a direct current motor and a direct current generator for supplying current thereto, of a normally open main switch for establishing a low resistance supply circuit between said motor and said generator, a speed selecting device, an adjustable field regulator rendered operative upon closure of said main switch to effect operation of said motor at speeds determined by the setting of said speed selecting device, said eld regulator being movable out of a given normal position to strengthen the eld of said generator for acceleration of said motor, and being returnable to normal position to weaken the eld of said generator for deceleration of said motor -by regenerative braking, manually controlled means effective upon movement of Said eld regulator out of normal position to restore the same to such position for stopping 0f said motor by regenerative operation thereof, a second 'reclosing the Same at any time during a, drifting The following references are of record in the 1 2,433,130 1l t 11 d 9 f in d i manua ycon ro e means or open g sai man switch for a drifting stop of said motor and for REFERENCES CITED stop for slowdown of said motor by regenerative le 0f this patent! operation thereof, and means rendered operative UNITED STATES PATENTS upon opemng of vsaid main switch for a drlftlng stop for automatically returning said field regu- Number Name Date motor during the drifting period.

EIVIND U. LASSEN. 

